This invention applies in particular to the production of oxygen, nitrogen and argon by cryogenic distillation. Over the years numerous efforts have been devoted to the improvement of this production technique to lower the oxygen cost which consists mainly of the power consumption and the equipment cost.
It has been known that an elevated pressure distillation system is advantageous for cost reduction and when the pressurized nitrogen can be utilized, the power consumption of the system is also very competitive. It is useful to note that an elevated pressure system is characterized by the fact that the pressure of the lower pressure column being above 2 bar absolute. The conventional or low pressure process by contrast has its lower pressure column operating at slightly above atmospheric pressure.
The higher the pressure of the lower pressure column, the higher is the air pressure feeding the high pressure column and the equipment for both warm and cold portions of the plant is more compact resulting in significant cost reduction. However, the higher the pressure, the more difficult is the distillation process since the volatilities of the components present in the air (oxygen, argon, nitrogen etc) become closer to each other such that it would be more power intensive to perform the separation by distillation. Therefore the elevated pressure process is well suited for the production of low purity oxygen (&lt;98 mol. % purity) wherein the separation is performed between the easier oxygen-nitrogen key components instead of the much more difficult oxygen-argon key components. The volatilities of oxygen and argon are so close such that even at atmospheric pressure it would require high number of distillation stages and high reboil and reflux rates to conduct such separation. The elevated pressure process in the current configuration of today's state-of-the-art process cycles is not suitable or economical for high purity oxygen production (&gt;98 mol. % purity). Since the main impurity in oxygen is argon, the low purity oxygen production implies no argon production since over 50% of argon contained in the feed air is lost in oxygen and nitrogen products.
One object of the invention is to provide an elevated pressure process capable of high purity oxygen production and also argon production.
The new process described below applies the basic double-column process with sidearm argon column with some modifications to improve the distillation under elevated pressure to yield higher purity oxygen along with the argon by-product.
One example of the elevated pressure double-column process is described in U.S. Pat. No. 5,224,045.
U.S. Pat. No. 4,737,177 describes double column system with a sidearm argon column wherein a short column is added above the overhead condenser of this column to improve further the distillation process for oxygen and argon production.
U.S. Pat. No. 5,572,874 describes a low pressure distillation process with argon wherein the low pressure rectification column of a double column system operates at 2 bar pressure or lower. In this process, an argon-enriched vapor stream is withdrawn from the low pressure rectification column and is at least partially condensed in a reboiler-condenser which reboils oxygen separated in the argon column. One part of the resulting at least partially condensed argon-enriched stream is expanded through a valve to a lower pressure and is introduced into the argon column in which it is separated into argon and oxygen. Even with additional trays at the bottom of the argon column to distil oxygen product and with lower operating pressure, this process still yield an acceptable temperature approach of the overhead condenser thanks to the low pressure drop of the structured packing being utilized in the argon column.
U.S. Pat. No. 5,305,611 describes a low pressure distillation process with argon wherein the low pressure rectification column of a double column system operates at between 14.7 and 75 psia. In this process, an argon-enriched vapor stream is withdrawn from the low pressure rectification column and is condensed in a reboiler-condenser which reboils the argon column. The resulting condensed argon-enriched stream is expanded through a valve to a lower pressure and is introduced into the argon column in which it is separated to form the argon rich product. The bottom liquid of the low pressure column is sent back to the low pressure column. In this system all the product oxygen is recovered at the bottom of the low pressure column.
U.S. Pat. No. 5,245,832 discloses a process wherein a double-column system at elevated pressure is used in conjunction with a third column to produce oxygen, nitrogen and argon. In order to perform the distillation at elevated pressure a nitrogen heat pump cycle is used to provide the needed reboil and reflux for the system. In addition to the power required for the separation of argon and oxygen in the third column the heat pump cycle must also provide sufficient reflux and reboil for the second column as well such that the resulting recycle flow and power consumption would be high.
The new invention improves the distillation at elevated pressure by adding a crude argon column to the elevated pressure double-column column process to perform an efficient separation of argon and oxygen. In one embodiment (FIG. 1) compressed air free of impurities such as moisture and CO2 is fed to a high pressure column where it is separated into a nitrogen rich stream at the top and an oxygen rich stream at the bottom. At least a portion of the oxygen rich stream is fed to a short column to yield a second nitrogen rich stream at the top and a second oxygen rich stream at the bottom. This short column has a reboiler which exchanges heat with the argon enriched gas at or near the top of the argon column.
At least a portion of the second nitrogen rich stream and/or at least a portion of the second oxygen rich stream is/are fed to the low pressure column.
At least a portion of the second oxygen rich stream is vaporized in the overhead condenser of the argon column and this vaporized stream and/or the non-vaporized portion is/are fed to the low pressure column.
The low pressure column separates its feeds into a third oxygen rich stream at the bottom and a third nitrogen rich stream at the top. At least a portion of the third oxygen rich stream is recovered as oxygen product in gaseous and/or liquid form.
An oxygen and argon containing gaseous stream is removed at an intermediate tray of the low pressure column. This oxygen-argon containing stream is at least partially condensed at the bottom reboiler of the argon column. A portion of this partially condensed oxygen-argon containing stream is fed to the argon column. An argon enriched stream is recovered at the top of the argon column and a fourth oxygen rich stream at the bottom of the crude argon column. At least a portion of the fourth oxygen rich stream is recovered as oxygen product.
According to an object of the invention, there is provided a process for production of oxygen enriched fluid and argon enriched fluid by cryogenic distillation of air comprising the steps of:
a) sending a feed stream containing nitrogen, oxygen and argon to a main column system wherein it is separated by cryogenic distillation; PA1 b) removing an argon containing gaseous stream from a column of the main column system, said column operating at a pressure of at least 2 bar abs., and at least partially condensing the argon containing gaseous stream; PA1 c) sending at least part of the at least partially condensed argon containing gaseous stream to an intermediate point of an argon column; and PA1 d) removing an argon enriched product stream from the top of the argon column and a first oxygen enriched product stream from the bottom of the argon column. According to optional features of the process, PA1 the argon containing gaseous stream condenses by indirect heat exchange with liquid at the bottom of the argon column. PA1 part of the at least partially condensed argon containing gaseous stream is sent to the main column system. PA1 the main column system comprises a high pressure column and a low pressure column, the argon containing gaseous stream being removed from the low pressure column. PA1 a stream containing nitrogen, oxygen and argon is expanded in a turbine and sending the expanded stream to the low pressure column. PA1 oxygen enriched liquid is sent from the high pressure column to a top condenser of the argon column. PA1 the oxygen content of the oxygen enriched liquid is enriched following removal from the high pressure column and before sending it to the argon column top condenser. PA1 a second oxygen enriched product stream is removed from the low pressure column. PA1 the first and second oxygen enriched product stream are mixed to form a mixed stream and the mixed stream is vaporized in a heat exchanger. PA1 the first and second oxygen enriched streams are mixed in the argon column and pumping the oxygen enriched stream removed from the argon column to a desired pressure. PA1 nitrogen enriched gas is removed from the high pressure and/or low pressure column. PA1 the argon containing gaseous stream contains between 3 and 20 mol. % argon. PA1 the argon containing gaseous stream is withdrawn at point between 2 and 12 theoretical trays above the bottom of the low pressure column.
The low pressure column in this process is defined as a column which operates at a pressure at its top of at least 2 bar abs. or higher.
FIGS. 1-3 show schematically installations which may be operated using the process according to the invention.